The present invention relates to apparatus for the induced infusion of a liquid from a flexible liquid bag, such as involved during a therapeutic treatment or a surgical operation of the patient.
In most cases, liquids are administered to patients by gravity-infusion, but there are a number of situations wherein induced infusion is required. At the present time, this is usually done by including a bag-presser which includes a bladder inflatable by a pumping bulb, and a flexible non-elastic sleeve for retaining the liquid bag firmly pressed against the inflated bladder such that the pressure of the bladder is transferred to the liquid bag to induce the infusion therefrom. An example of such a bag-presser is illustrated in U.S. Pat. No. 3,153,414.
One of the main drawbacks in the use of the known bag-pressers of this type is that the pressure applied by the inflated bladder to the liquid bag changes quite substantially during the decrease in volume of the bag unless the bladder is periodically reinflated to compensate for the loss in volume of the bag. For example, in a typical bag-presser commonly used today, wherein the bladder is inflated to a pressure of 300 mm Hg, a liquid bag of a volume of 500 cc is usually pressurized slightly greater than 300 mm Hg at the start when its initial volume is equal to 500 cc, but its pressure drops to about 300 mm Hg when its volume reaches 400 cc, to about 270 mm Hg when its volume reaches 300 cc, to about 230 mm Hg when its volume reaches 200 cc, to about 200 mm Hg when its volume reaches 100 cc, and to about 180 mm Hg when its volume reaches 50 cc. This drop in pressure during the decrease in volume of the liquid bag may result in the required amount of the liquid not being administered at the required rate or within the required period of time. To avoid this, the infusion procedure must be continuously monitored and the bag-presser bladder reinflated as necessary to maintain the desired rate of infusion, but such continuous monitoring is very demanding of the nurse's time and therefore cannot always be provided.
An object of the present invention is to provide apparatus for the induced infusion of a liquid from a liquid bag which provides a substantially uniform pressure to the liquid bag, and therefore a substantially uniform rate of infusion, until substantially the complete contents of the bag are dispensed.
Another object of the invention is to provide apparatus of the above type having a number of further advantages as will be described more particularly below.